A) Field of the Invention
The present invention relates to a semiconductor device and its manufacture method, and more particularly to a method of manufacturing a MOS FET whose gate electrode, source and drain regions are implanted with impurities and a semiconductor device fabricated by such a method.
B) Description of the Related Art
Polysilicon or amorphous silicon is used as the material of the gate electrode of a MOSFET. Impurities are implanted into a gate electrode either by implanting p- or n-type impurities into a silicon film to be used as the gate electrode before the silicon film is patterned or by patterning a silicon film and thereafter implanting impurities into the source and drain regions and the gate electrode at the same time.
If a silicon film is patterned after impurities are implanted into the silicon film, it is difficult to control the cross sectional shape of the gate electrode.
FIG. 5A shows examples of the cross sectional shape of a gate electrode. An element separation insulating film 501 is formed in the surface layer of a silicon substrate 500. Gate electrodes 502 and 503 are formed in active regions defined by the element separation insulating film 501. The gate electrodes 502 and 503 are formed by patterning a silicon film without annealing the phosphorous (P) and boron (B) implanted areas of the silicon film.
The silicon film implanted with boron and patterned can form the gate electrode 503 having generally a rectangular shape. In contrast, the silicon film implanted with phosphorous and patterned forms the gate electrode 502 having a cross sectional shape with its middle region being constricted.
FIG. 5B is a cross sectional view of a gate electrode formed by implanting impurities into a silicon film and annealing before the film is patterned. In the area where boron was implanted, a gate electrode 503A having a rectangular cross section can be obtained, whereas in the area where phosphorous was implanted, the cross sectional shape of a gate electrode 502A is likely to have a trapezoidal shape with a bell-bottom.
Variations of the cross sectional shape raise no serious problem if a minimum processing size is relatively large. As the degree of integration becomes high, a silicon oxide film or a silicon oxynitride film formed on the surface of a gate electrode is very thin. If there are variations of the cross sectional shape of a gate electrode, it is difficult to form a very thin film with good reproductivity.
If after a silicon film is patterned, impurities are implanted into source and drain regions and a gate electrode at the same time, it is not possible to independently control the impurity concentrations of the source and drain regions and the gate electrode. In order to suppress depletion in a gate electrode, it is desired to set the dose of ion into the gate electrode, for example, to 7×1015 cm−2 or more.
In this case, the dose of the source and drain regions is also 7×1015 cm−2. As the dose of the source and drain regions is increased to the same degree as that of the gate electrode, the source and drain regions extend just under the gate electrode so that the influence of short channel effects becomes high.